1. Field of the Invention
The present invention relates to a surface acoustic wave filter and, more particularly, to a surface acoustic wave filter including a surface acoustic wave resonator connected in series with a surface acoustic wave filter section.
2. Description of the Related Art
Surface acoustic wave filters are commonly used as bandwidth filters in an RF stage in a mobile telephone.
For example, Japanese Unexamined Patent Application Publication No. 7-30367 discloses this sort of surface acoustic wave filter. FIG. 41 illustrates the arrangement of the electrodes of the disclosed surface acoustic wave filter. In a surface acoustic wave filter 2001, a surface acoustic wave resonator 2003 is connected in series with a serially coupled 3-IDT (interdigital transducer) resonator type surface acoustic wave filter section 2002. The antiresonance frequency of the surface acoustic wave resonator 2003 is on a high frequency side of the passband of the longitudinally coupled resonator type surface acoustic wave filter section 2002, while the resonance frequency of the surface acoustic wave resonator 2003 is within the passband. Since the antiresonance frequency of the surface acoustic wave resonator 2003 is on a high frequency side of the passband of the longitudinally coupled resonator type surface acoustic wave filter section 2002, attenuation in the vicinity of the high frequency side of the passband is increased. Since the resonance frequency falls within the passband, the transmission characteristic within the passband does not substantially suffer from degradation.
If the surface acoustic wave filter 2001 is used as a wideband filter such as a DSC filter having a wide passband width, the VSWR (Voltage-Standing-Wave Ratio) in a high-frequency region of the passband is not high enough. The effect of parasitic capacitances generated in a piezoelectric substrate and a package is predominant in a high frequency region of the filter, and the impedance of the filter is capacitive if a wideband filter feature is implemented.
This trend becomes pronounced in the high frequency region of the passband. A threefold mode filter is typically used to widen the band width. However, the frequency separation between three resonance modes naturally increases in an attempt to achieve the wideband feature. On the other hand, to balance impedance matching within the passband, the impedance matching is performed on a center resonance mode of the three resonance modes. The two remaining resonance modes in a low frequency and a high frequency region are thus not matched. In high frequency applications in particular, the effect of capacitance in the resonance mode in the high frequency region is pronounced. The impedance of the filter is inductive on the resonance mode in the low frequency region while being capacitive on the resonance mode in the high frequency region. The resonance mode in the high frequency region suffers from the effect of capacitance and thus, experiences the above-mentioned problem.
To overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave filter which includes a surface acoustic wave resonator connected in series with a surface acoustic wave filter section and which has an outstanding VSWR in a wide passband.
In a first preferred embodiment of the present invention, a surface acoustic wave filter includes a surface acoustic wave filter section including a piezoelectric substrate, at least two IDTs arranged in a direction of propagation of a surface acoustic wave on the piezoelectric substrate, each of the IDTs having a narrow-pitched electrode finger portion at one end of the IDT including electrode fingers arranged at a pitch narrower than that of electrode fingers in the remaining portions thereof, at an area where the IDTs are adjacent to each other, and at least one surface acoustic wave resonator connected in series between the surface acoustic wave filter section and one of an input signal terminal and an output signal terminal. The surface acoustic wave filter section is a threefold mode surface acoustic wave filter section of a longitudinally coupled resonator type, and impedance of the surface acoustic wave filter section is capacitive in a resonance mode in the highest frequency region of three resonance modes. The resonance point of the surface acoustic wave resonator is within a passband of the surface acoustic wave filter section, and the antiresonance point of the surface acoustic wave resonator is in the vicinity of the high frequency region of the passband of the surface acoustic wave filter section. The surface acoustic wave resonator is arranged such that the impedance in the resonance mode in the highest frequency region is close to an impedance matching point.
Preferably, at least a portion of the IDT is weighted in an area where a plurality of IDTs are disposed adjacent to each other in the surface acoustic wave filter section. Weighting the portion of the IDT improves the out-of-passband characteristics of the filter. An unbalance-balance converting function compensates for an amplitude deviation and a phase shift from a phase difference of 180xc2x0 in a signal output from a circuit between an unbalanced signal terminal and one of the balanced signal terminals with respect to an input signal to a circuit between the unbalanced signal terminal and the other of the balanced signal terminals.
Preferably, the surface acoustic wave filter section includes an odd number of IDTs, and at least one surface acoustic wave resonator is connected between the surface acoustic wave filter section and one of the input signal terminal and the output signal terminal having a greater number of IDTs connected thereto. The VSWR is thus further improved.
Preferably, the surface acoustic wave resonators include at least one surface acoustic wave resonator connected in series between the surface acoustic wave filter section and the input signal terminal, and at least one surface acoustic wave resonator connected in series between the surface acoustic wave filter section and the output signal terminal. The VSWR is thus further improved.
In a second preferred embodiment of the present invention, a surface acoustic wave filter includes a surface acoustic wave filter section including a piezoelectric substrate, at least two IDTs arranged in a direction of propagation of a surface acoustic wave on the piezoelectric substrate, and a reflector arranged between adjacent IDTs and having a plurality of electrode fingers at an electrode finger pitch that is different from the finger pitch of the IDTs, and at least one surface acoustic wave resonator connected in series between the surface acoustic wave filter section and one of an input signal terminal and an output signal terminal. The surface acoustic wave filter section is a longitudinally coupled resonator type, threefold mode surface acoustic wave filter section, and impedance of the surface acoustic wave filter section is capacitive in a resonance mode in the highest frequency region of the three resonance modes. The resonance point of the surface acoustic wave resonator is within a passband of the surface acoustic wave filter section, and the antiresonance point of the surface acoustic wave resonator is in the vicinity of the high frequency region of the passband of the surface acoustic wave filter section. The surface acoustic wave resonator is arranged such that the impedance in the resonance mode in the highest frequency region is close to an impedance matching point.
Preferably, the surface acoustic wave filter section includes an odd number of IDTs, and at least one surface acoustic wave resonator is connected between the surface acoustic wave filter section and one of the input signal terminal and the output signal terminal having a greater number of IDTs connected thereto. The VSWR is thus further improved.
The surface acoustic wave resonators preferably include at least one surface acoustic wave resonator connected in series between the surface acoustic wave filter section and the input signal terminal, and at least one surface acoustic wave resonator connected in series between the surface acoustic wave filter section and the output signal terminal. The VSWR is thus further improved.
Preferably, the surface acoustic wave filter includes a plurality of surface acoustic wave resonators connected between the surface acoustic wave filter section and at least one of the input signal terminal and the output signal terminal.
At least one of the input signal terminal and the output signal terminal preferably includes a pair of balanced signal terminals.
Preferably, the input signal terminal and the output signal terminal function is an unbalance-balance converter with one of the input signal terminal and output signal terminal being a balanced signal terminal, and the other of the input signal terminal and output signal terminal being an unbalanced signal terminal.
Preferably, the surface acoustic wave filter section includes a first surface acoustic wave filter block and a second surface acoustic wave filter block, each outputting an output signal different in phase by 180xc2x0 from a signal input thereto. Ends of the first and second surface acoustic wave filter blocks are connected together and function as an unbalanced signal terminal, and the other ends of the first and second surface acoustic wave filter blocks and function as balanced signal terminals.
Preferably, the surface acoustic wave filter section includes a single surface acoustic wave filter block, terminals of the surface acoustic wave filter block function as a pair of balanced signal terminals, and the other terminal of the surface acoustic wave filter block functions as an unbalanced signal terminal.
At least one IDT of the surface acoustic wave filter section preferably includes first and second separate IDT sections split in a direction across the electrode finger or in a direction of propagation of a surface acoustic wave.
The surface acoustic wave resonator and the surface acoustic wave filter section are preferably provided on the same piezoelectric substrate. The surface acoustic wave resonator is thus produced as a single chip component.
Preferably, the surface acoustic wave filter further includes a case plate, wherein the piezoelectric substrate is mounted on the case plate such that the side of the piezoelectric substrate having the surface acoustic wave filter section and the surface acoustic wave resonator provided thereon faces the case plate. In accordance with preferred embodiments of the present invention, a surface acoustic wave filter device having a surface acoustic wave filter element mounted on the case plate is preferably manufactured using flip-chip technique.
In a third preferred embodiment of the present invention, a communication apparatus includes the surface acoustic wave filter according to preferred embodiments of the present invention. The communication apparatus of the present invention includes a surface acoustic wave filter having a wide bandwidth and excellent VSWR.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.